The invention relates generally to data storage systems, and in particular, to data storage systems with remote data mirroring capability.
Given the importance of the availability of information, several techniques have been developed for providing enhanced reliability and availability of data stored in a data storage system. Once such technique is remote data mirroring. In a “mirrored” system, the data stored on one data storage system is replicated on another data storage system, preferably at a geographically remote site. Thus, if one or more storage devices on one of the data storage systems fails, or a catastrophic system failure should occur, the data is readily available in the form of a mirrored copy from the remote data storage system.
Devices on a data storage system (or source) that are mirrored on the same remote, data storage system (or target) are referred to as a device group. Likewise, devices on the target that serve or mirror devices on the same source are referred to as a device group. Device groups are used, for example, to insure the consistency of blocks of data too large to be stored in a single area, during planned or unplanned interruptions such as link failures or planned back-ups. Device groups typically span a number of physical and logical volumes, and, in some instances, as when data striping is used, several device groups may be mapped to different portions of a physical or logical volume.
Typically, in a remote data mirroring environment, the source and target device groups are arranged as pairs on any two systems and the source/target device group pairs are connected by dedicated data links (e.g., ESCON links) or switched data links (e.g., switched Fibre Channel links). The data links support writes during data backup operations and reads during data recovery operations.
Such point-to-point and switched data link topologies have distance limitations. To negotiate long distances, the ESCON (or Fibre Channel) data links (connected to data ports of the data storage systems, local and remote) are coupled via a leased line (such as T3) or an IP network. There are significant drawbacks associated with these types of long distance solutions, however. For example, the T3 link is extremely expensive and very slow compared to the ESCON or Fibre Channel links. In addition, because connections using these solutions span diverse network protocols and interfaces, some type of adapter box must be used to translate between diverse protocols and interfaces of, say, ESCON and T3, or ESCON and IP. Typically, the adapter box is designed, configured and managed by some entity other than the data storage system supplier. This means that some aspects of the data storage system's performance are either dictated by the adapter box (for example, delays due to the buffer constraints or encapsulation, availability of IP services), or the quality of the IP line, for example, an IP connection provided by an Internet Service Provider, and are therefore not within the control of the data storage system supplier. Moreover, the design of the adapter boxes, in particular, those supporting FC-to-IP services, can be quite complex, making networks of remotely-connected data storage systems that employ such boxes expensive from a field service perspective.